The present invention relates generally to methods, apparatus and compositions for making plastic lenses.
It is conventional in the art to produce optical lenses by thermal curing techniques from the polymer of diethylene glycol bis(allyl)-carbonate (DEG-BAC).
The polymer of DEG-BAC exhibits desirable optical and mechanical properties. These properties include high light transmission, high clarity, and high index of refraction together with high abrasion and impact resistance. These properties in the past made DEG-BAC one of the leading monomers in the manufacture of high quality lenses, face shields, sun and safety glasses. Other properties of DEG-BAC, however, such as its slow rate of polymerization make it an undesirable monomer in the manufacture of these items. Moreover, DEG-BAC, without any additives or comonomers produces a very hard but somewhat brittle polymer that is very prone to cracking. In addition, DEG-BAC, without additives adheres very tightly to the lens forming molds, often leading to cracking of the molds.
In addition, the thermal curing techniques for polymerizing DEG-BAC to produce optical lenses have several disadvantages and drawbacks. One of the most significant drawbacks is that it may take approximately 12 hours to produce a lens according to thermal curing techniques. A lens forming mold, therefore, can produce at most two lenses per day.
Moreover, thermal curing techniques employ a thermal catalyst so that a polymerizable mixture of DEG-BAC and catalyst will slowly polymerize even while refrigerated. The polymerizable mixture therefore has a very short shelf life and must be used within a short time or it will harden in its container.
Furthermore, the thermal catalysts utilized according to the thermal curing techniques are quite volatile and dangerous to work with requiring extreme care in handling.
U.S. Pat. Nos. 3,038,210 and 3,222,432 disclose methods for producing lenses by thermal curing techniques.
Optical lenses have also been produced by photocuring techniques. For example, U.S. Pat. No. 4,166,088 discloses a method for producing a plastic lens by directing rays of ultraviolet light against a mold having a lens forming material disposed in a cavity within the mold.
U.S. Pat. No. 4,298,005 discloses an apparatus for generating ultraviolet light having a wavelength in the range of 320 to 450 nm for hardening plastics.
In general, photochemical and thermal curing systems are analogous except that light instead of heat is the main driving force for the polymerization reaction. Curing of a lens by ultraviolet light, however, presents certain problems that must be overcome to produce a viable lens. Such problems include yellowing of the lens, cracking of the lens or mold, optical distortions in the lens and premature release of the lens from the mold.
The present invention is directed to methods, apparatus and compositions for making plastic lenses that overcome the disadvantages and drawbacks of the prior art.